Seed treatment trials hit corn leafhopper

Initial results of research trials using seed treatments to control corn leafhopper, a serious San Joaquin Valley pest of corn that eludes management with sprayed insecticides, were recently disclosed.

Charles Summers, University of California entomologist at the Kearney Research and Education Center, Parlier, detailed his findings to date during an alfalfa and forage field day there.

Summers said the best performance against corn leafhopper so far was with seed treated with Bayer CropScience’s Poncho 1250 (1.25 milligram active ingredient per seed), which showed a 10 percent survival of the insect 32 days after emergence of silage corn in the trials during the past summer. The untreated check showed a 90 percent survival of the leafhopper.

Also in the trials were Poncho 250 (0.25 milligram a.i. per seed) and Syngenta’s Cruiser 1250 and Cruiser 250. Summers said he is in the process of analyzing additional results and will release them later.

Adult leafhoppers, laboratory-reared for the trials, were placed in cages over young plants from corn seeded first in April and then in a second planting seeded in late June.

The seed treatments are both systemics in the neo-nicotinoid insecticide class. Although used for other corn pests, neither is registered for use on corn leafhopper, and Summers said there’s been no previous research on how they affect corn leafhopper.

“We’ve also sent in silage samples from the trial for testing for the presence of corn stunt to see if the seed treatments had any effect on the disease,” he said.

Although the leafhopper inflicts damage by its direct feeding which damages leafs and causes soot mold, the greater economic loss is from the corn stunt caused by the bacteria-like Spiroplasma kunkelii it carries from one plant to the next.

In addition to stunting the corn, as its name implies, the disease leads to multiple ears that fail to fill. Infected leaves take on a reddish color in the fall. The disease goes to silage and other types of corn.

Although corn leafhopper has long been known in the SJV, only since 1996 have outbreaks of it been annual events with increasing intensity in the southern part of the Valley. In some fields losses to it have reached as high as 50 percent.

The insect, brownish or tan in color and less than one-eighth inch in length, resides in the whorls of corn stalks. It can overwinter in the adult form in grassy vegetation, but the preferred winter host is alfalfa, which it does not feed on. As temperatures warm in the spring, it moves to newly planted cornfields.

Summers also reported on South American bean thrips on alfalfa in the SJV. Quite different from Western flower thrips, which cause only minor crinkling of alfalfa foliage, bean thrips can do considerable damage that resembles, from a distance, moisture stress. A closer look at plants reveals blasted terminals and large whitened areas of epidermis stripped away.

An occasional problem in Southern California for a number of years, bean thrips have caused concern in alfalfa in the SJV only in the past year, he said.

Bean thrips adults are black with white bands on the wings. Nymphs are yellow with pinkish or reddish markings.

“The dead give-away for South American bean thrips is the huge amount of fecal material they leave on the plant. You don’t see this with Western flower thrips,” he said.

In an account of vertebrate pest management in alfalfa, Roger Baldwin, UC IPM wildlife pest management advisor at Parlier, said habitat modification is high on the list of alternatives for control of many vertebrate pests, with the exception of deer.

He recommended that the pest species be identified before considering what control options are available and effective.

Pocket gophers, for example, are the chief vertebrate pest of alfalfa, although rarely seen, they leave distinguishing mounds of earth with openings plugged. Ground squirrels, on the other hand, are readily observed in the daytime and their burrows are open.

Regulations for endangered species and other matters under county agricultural commissioners’ offices and California Department of Fish and Game authority should be observed.

“One method of habitat modification is removal of piles of brush from around your alfalfa fields. This can reduce one of the preferred burrowing sites for ground squirrels and increase the effectiveness of other management options you might use.”

Poison baits may be effective, although some are restricted use pesticides and require certification to use. For example, zinc phosphide is one of few materials for control of field voles in alfalfa. Baits placed around the edges of alfalfa fields can control ground squirrels, although they are not particularly successful for hares and rabbits.

Generally, he recommended, lower concentrations of poisonous baits should be used first and stronger materials used if needed.

Another good measure for ground squirrels and pocket gophers is burrow fumigation with poisonous gasses such as aluminum phosphide, which is a restricted material. Gas cartridges also work for ground squirrels and are not restricted, although they are not effective against pocket gophers.

“Trapping may be a good option for small populations of ground squirrels and pocket gophers, but it is less effective for large populations due to the time involved in using it,” Baldwin said. Traps may also be a good idea for follow-up after baiting.

Exclusionary devices for rabbit control such as fencing are quite expensive for alfalfa, and Baldwin suggested that before fencing is installed an assessment be made of the cost of the damage versus the cost of the fencing.

A propane-powered or other type of frightening device might scare off deer invading alfalfa, but would not be broadly reliable.

Shooting may be an answer for pests like rabbits or ground squirrels, but is labor-intensive. If deer are a significant problem, allowing hunting on your land or getting a depredation permit could be alternatives.

“A final step is to monitor your fields,” Baldwin said, noting that new gopher mounds, for example, indicate a treatment has not been successful.

“This is particularly important after a particular management strategy to see how effective it was. Then you can take further steps if needed to reduce those populations.”

Worthwhile too is keeping records of what was done, the weather conditions, and the success of one control or another, for a compilation of details to learn which method works best when, Baldwin said.